I run the latest version of P3D in vanilla only with the FSLabs 320 installed. My hardware is my laptop as my desktop is not available atm. The laptop has an MX150 GPU and an I5-8250U CPU (4C 8T @ 3.4 boost). It definitely can handle P3D in vanilla with low settings; I know this because it could before. Now I get some weird stuff going on.

When the laptop is in normal power mode the CPU clocks in an 800 MHz - 3.4 GHz range. Perfectly normal. When stress testing the CPU, I get a continuously 3.4 GHz clock.

As soon as I run P3D, the maximum core clock drops to 1.2-1.4 GHz. Even when using high-performance mode/stress testing (basically doing anything to force a 100% speed). As soon as I close P3D again the CPU rocks back up to 3.4 GHz.

I think I do not have to mention how ridiculous this is.

As usual... all drivers are up to date, tried the know troubleshooting steps, reinstall, and you name it. (using P3D for years now and I'm getting pretty familiar with the pain it brings)

EDIT:
Should have mentioned that it happens with any aircraft so it's not the FSLabs.

Ah, I meant did it also happen with a virgin install of P3D? The reason I'm asking like this is that installing the FSLabs may have put some files on there that didn't get removed when you uninstalled again....

I found out as I started to explore overclocking that my HP basically locked my chip to minimum clock speed (even though advertised to OC). I can set it to overclock and it will run at that speed until any stress and then the electric current throttles the chip back to minimum. Maybe your laptop is doing something similar.

I'm 100% positive that it's definitely not a thermal throttle.
If HWmonitor is to be trusted, which is always right, my CPU dips around 50-60°C. Also, the downclock happens immediately after P3D is launched, and a stress test revealed no throttling even after 25 min.
SO no it has to be P3D as my CPU works perfectly fine in all other programs.

Do a search for i5-8250U throttling and see what you find. There are several reports that indicate it's not just core temp that slows this CPU down. Depending on the laptop, it can even be an under capacity power supply.

I appreciate all the answer, but again the CPU works just fine any application, game, render engine or stress test.
It always clocks at maximum speed, and no issue is present.
The ONLY application suffering from this issue is P3D. So as I said I appreciate the answers regarding the throttling but it can't be it.

That only goes to prove that no other application than P3D loads up your laptop and places higher demands on it than any other application. End of story.

Jorgen

Please don't take it the wrong way, but you do know that a stress test is putting the CPU to 100% load? (I've used P95 and Cinebench.)
That means that even IF P3D would use 100%, which it doesn't by the way, I should get the same result.
That's how percentage works.

I am talking about the whole system, not just the CPU itself. As another gentleman pointed out, it may be the power supply in this laptop that triggers this issue.

Remember that in a laptop the cooling can be an issue. Not only for the CPU, but also for the graphics adapter and the power supply. So, if there is a cooling problem in any of these, the whole system goes into thermal protection mode (I won't say thermal shutdown, but at least slowdown).

In order to slow the clock rate down, the BIOS of the system has to be accessed. I do not know of any normal consumer-grade application, including P3D, that does that. But the thermal protection system resides there.